Leflunomide is a pyrimidine synthesis inhibitor belonging to the DMARD (disease-modifying antirheumatic drug) class of drugs, which are chemically and pharmacologically very heterogeneous. Leflunomide was approved by FDA and in many other countries (e.g., Canada, Europe) in 1999.

This compound belongs to the anilides. These are organic heterocyclic compounds derived from oxoacids RkE(=O)l(OH)m (l not 0) by replacing an OH group by the NHPh group or derivative formed by ring substitution.

Pharmacology

Indication

For the management of the signs and symptoms of active rheumatoid arthritis (RA) to improve physical function and to slow the progression of structural damage associated with the disease. Has also been used for the prevention of acute and chronic rejection in recipients of solid organ trasnplants and is designated by the FDA as an orphan drug for this use.

Pharmacodynamics

Leflunomide is a pyrimidine synthesis inhibitor indicated in adults for the treatment of active rheumatoid arthritis (RA). RA is an auto-immune disease characterized by high T-cell activity. T cells have two pathways to synthesize pyrimidines: the salvage pathways and the de novo synthesis. At rest, T lymphocytes meet their metabolic requirements by the salvage pathway. Activated lymphocytes need to expand their pyrimidine pool 7- to 8-fold, while the purine pool is expanded only 2- to 3-fold. To meet the need for more pyrimidines, activated T cells use the de novo pathway for pyrimidine synthesis. Therefore, activated T cells, which are dependent on de novo pyrimidine synthesis, will be more affected by leflunomide's inhibition of dihydroorotate dehydrogenase than other cell types that use the salvage pathway of pyrimidine synthesis.

Mechanism of action

Leflunomide is a prodrug that is rapidly and almost completely metabolized following oral administration to its pharmacologically active metabolite, A77 1726. This metabolite is responsible for essentially all of the drug's activity in-vivo. The mechanism of action of leflunomide has not been fully determined, but appears to primarily involve regulation of autoimmune lymphocytes. It has been suggested that leflunomide exerts its immunomodulating effects by preventing the expansion of activated autoimmune lymphocytes via interferences with cell cycle progression. In-vitro data indicates that leflunomide interferes with cell cycle progression by inhibiting dihydroorotate dehydrogenase (a mitochondrial enzyme involved in de novo pyrimidine ribonucleotide uridine monophosphate (rUMP)synthesis) and has antiproliferative activity. Human dihydroorotate dehydrogenase consists of 2 domains: an α/β-barrel domain containing the active site and an α-helical domain that forms a tunnel leading to the active site. A77 1726 binds to the hydrophobic tunnel at a site near the flavin mononucleotide. Inhibition of dihydroorotate dehydrogenase by A77 1726 prevents production of rUMP by the de novo pathway; such inhibition leads to decreased rUMP levels, decreased DNA and RNA synthesis, inhibition of cell proliferation, and G1 cell cycle arrest. It is through this action that leflunomide inhibits autoimmune T-cell proliferation and production of autoantibodies by B cells. Since salvage pathways are expected to sustain cells arrested in the G1 phase, the activity of leflunomide is cytostatic rather than cytotoxic. Other effects that result from reduced rUMP levels include interference with adhesion of activated lymphocytes to the synovial vascular endothelial cells, and increased synthesis of immunosuppressive cytokines such as transforming growth factor-β (TGF-β). Leflunomide is also a tyrosine kinase inhibitor. Tyrosine kinases activate signalling pathways leading to DNA repair, apoptosis and cell proliferation. Inhibition of tyrosine kinases can help to treating cancer by preventing repair of tumor cells.

The active metabolite is eliminated by further metabolism and subsequent renal excretion as well as by direct biliary excretion. In a 28 day study of drug elimination (n=3) using a single dose of radiolabeled compound, approximately 43% of the total radioactivity was eliminated in the urine and 48% was eliminated in the feces. It is not known whether leflunomide is excreted in human milk.
Many drugs are excreted in human milk, and there is a potential for serious adverse reactions in nursing infants from leflunomide.

Immunosuppressants like bleomycin may enhance the adverse/toxic effect of Leflunomide. Specifically, the risk for hematologic toxicity such as pancytopenia, agranulocytosis, and/or thrombocytopenia may be increased. Consider eliminating the use of a leflunomide loading dose in patients who are receiving other immunosuppressants in order to reduce the risk for serious adverse events such as hematologic toxicity.

Immunosuppressants such as carboplatin may enhance the adverse/toxic effect of leflunomide. Specifically, the risk for hematologic toxicity such as pancytopenia, agranulocytosis, and/or thrombocytopenia may be increased. Consider eliminating the use of a leflunomide loading dose in patients who are receiving other immunosuppressants in order to reduce the risk for serious adverse events such as hematologic toxicity. Also, patients receiving both leflunomide and another immunosuppressive medication should be monitored for bone marrow suppression at least monthly throughout the duration of concurrent therapy.

Immunosuppressants such as carmustine may enhance the adverse/toxic effect of leflunomide. Specifically, the risk for hematologic toxicity such as pancytopenia, agranulocytosis, and/or thrombocytopenia may be increased. Consider eliminating the use of a leflunomide loading dose in patients who are receiving other immunosuppressants in order to reduce the risk for serious adverse events such as hematologic toxicity. Also, patients receiving both leflunomide and another immunosuppressive medication should be monitored for bone marrow suppression at least monthly throughout the duration of concurrent therapy.

Immunosuppressants such as chlorambucil may enhance the adverse/toxic effect of leflunomide. Specifically, the risk for hematologic toxicity such as pancytopenia, agranulocytosis, and/or thrombocytopenia may be increased. Consider eliminating the use of a leflunomide loading dose in patients who are receiving other immunosuppressants in order to reduce the risk for serious adverse events such as hematologic toxicity. Also, patients receiving both leflunomide and another immunosuppressive medication should be monitored for bone marrow suppression at least monthly throughout the duration of concurrent therapy.

Immunosuppressants such as cisplatin may enhance the adverse/toxic effect of leflunomide. Specifically, the risk for hematologic toxicity such as pancytopenia, agranulocytosis, and/or thrombocytopenia may be increased. Consider eliminating the use of a leflunomide loading dose in patients who are receiving other immunosuppressants in order to reduce the risk for serious adverse events such as hematologic toxicity. Also, patients receiving both leflunomide and another immunosuppressive medication should be monitored for bone marrow suppression at least monthly throughout the duration of concurrent therapy.

Immunosuppressants such as cladribine may enhance the adverse/toxic effect of leflunomide. Specifically, the risk for hematologic toxicity such as pancytopenia, agranulocytosis, and/or thrombocytopenia may be increased. Consider eliminating the use of a leflunomide loading dose in patients who are receiving other immunosuppressants in order to reduce the risk for serious adverse events such as hematologic toxicity. Also, patients receiving both leflunomide and another immunosuppressive medication should be monitored for bone marrow suppression at least monthly throughout the duration of concurrent therapy.

Immunosuppressants such as clofarabine may enhance the adverse/toxic effect of leflunomide. Specifically, the risk for hematologic toxicity such as pancytopenia, agranulocytosis, and/or thrombocytopenia may be increased. Consider eliminating the use of a leflunomide loading dose in patients who are receiving other immunosuppressants in order to reduce the risk for serious adverse events such as hematologic toxicity. Also, patients receiving both leflunomide and another immunosuppressive medication should be monitored for bone marrow suppression at least monthly throughout the duration of concurrent therapy.

Bile Acid Sequestrants may decrease serum concentrations of the active metabolite(s) of Leflunomide. Unless using cholestyramine (or another bile acid sequestrant) together with leflunomide to intentionally enhance the removal/elimination of leflunomide, consider using an alternative to the bile acid sequestrants whenever possible. Separating the administration of these agents is unlikely to be an effective means of avoiding the interaction.

Immunosuppressants may enhance the adverse/toxic effect of Leflunomide. Specifically, the risk for hematologic toxicity such as pancytopenia, agranulocytosis, and/or thrombocytopenia may be increased. Consider eliminating the use of a leflunomide loading dose in patients who are receiving other immunosuppressants in order to reduce the risk for serious adverse events such as hematologic toxicity. Also, patients receiving both leflunomide and another immunosuppressive medication should be monitored for bone marrow suppression at least monthly throughout the duration of concurrent therapy.

Leflunomide may experience an increase in toxicity and adverse effects (ie. pancytopenia, agranulocytosis, thrombocytopenia). It is recommended to adjust therapy by forgoing a leflunomide loading dose, and to monitor for bone marrow suppression monthly.

Vinblastine may increase the adverse/toxic effects of Leflunomide. This may increase the risk of hematologic toxicities such as pancytopenia, agranulocytosis and thrombocytopenia. In patients receiving Vinblastine, consider eliminating the loading dose of Leflunomide. Monitor for bone marrow suppression at least monthly during concomitant therapy.

Vincristine may increase the adverse/toxic effects of Leflunomide. This may increase the risk of hematologic toxicities such as pancytopenia, agranulocytosis and thrombocytopenia. In patients receiving Vincristine, consider eliminating the loading dose of Leflunomide. Monitor for bone marrow suppression at least monthly during concomitant therapy.

Vinorelbine may increase the adverse/toxic effects of Leflunomide. This may increase the risk of hematologic toxicities such as pancytopenia, agranulocytosis and thrombocytopenia. In patients receiving Vinorelbine, consider eliminating the loading dose of Leflunomide. Monitor for bone marrow suppression at least monthly during concomitant therapy.